After reading the installation manual for the Worcester Highflow 440 i note that if the heating system has a volume of more than 100 litres it is going to need an extra external expansion vessel fitted. I have never installed a system this big before. Can anyone point me in the right direction of how I can work this out. I have tried finding the volumes held by radiators but can't find the figures. The system I am installing has 13 rads so I assume it is going to need to need an extra expansion vessel. But say the volume comes to 180 litres -how do I work out how much more expansion it needs?
Calculating system volume
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You should take the MI recomendations for expansion vessels and system volume with a pinch of salt. 
The question comes down to the difference between the system design pressure (normally around 1 Bar cold but dependent on the vessel position and height of the system) and the maximum hot pressure required.
Manufactures are often happy to have the system running at anything up to 2.5 bar hot. There has been a general reduction in boiler vessel size as manufacturers cut costs. This however results in a large pressure swing each time the system heats up and cools stressing components and leaving little margin before an overpressure situation arises and the safety valve opens.
Personanly I like to see a minimal difference between cold and hot pressures so will oversize the vessel accodingly since they are very little extra cost on the installation. This allows a large safey margin from overpresure (especially if the user sets the cold pressure a little higher than they should) and a longer time interval between topping up the system and checking the vessel pre-charge.
You can measure system volume by checking the radiator manufacturer tables - they normally qoute per section.
As a rough guide (assuming normal panel radiators) small 1 bed flats 2 or 3 buckets, mid terrace 3 beds say 5 or 6 buckets and larger 4 beds 10 to 12 buckets (bucket 10 litres). The required calculations are straight forward and vessel sizing calculators are available to save on the maths (try Altecnic).
As for your system although the boiler incorporates a relatively large vessel of 12 litres for a combi the boiler does have a large thermal store. I would suggest you add another 24 litre vessel since this size can still be fitted to a wall and is minimal extra cost. If the vessel pre-charge is set to 1 Bar and design cold pressure is 1 Bar this should rise to no more than 1.5 Bar hot with an extra 150 litre radiator volume.
The question comes down to the difference between the system design pressure (normally around 1 Bar cold but dependent on the vessel position and height of the system) and the maximum hot pressure required.
Manufactures are often happy to have the system running at anything up to 2.5 bar hot. There has been a general reduction in boiler vessel size as manufacturers cut costs. This however results in a large pressure swing each time the system heats up and cools stressing components and leaving little margin before an overpressure situation arises and the safety valve opens.
Personanly I like to see a minimal difference between cold and hot pressures so will oversize the vessel accodingly since they are very little extra cost on the installation. This allows a large safey margin from overpresure (especially if the user sets the cold pressure a little higher than they should) and a longer time interval between topping up the system and checking the vessel pre-charge.
You can measure system volume by checking the radiator manufacturer tables - they normally qoute per section.
As a rough guide (assuming normal panel radiators) small 1 bed flats 2 or 3 buckets, mid terrace 3 beds say 5 or 6 buckets and larger 4 beds 10 to 12 buckets (bucket 10 litres). The required calculations are straight forward and vessel sizing calculators are available to save on the maths (try Altecnic).
As for your system although the boiler incorporates a relatively large vessel of 12 litres for a combi the boiler does have a large thermal store. I would suggest you add another 24 litre vessel since this size can still be fitted to a wall and is minimal extra cost. If the vessel pre-charge is set to 1 Bar and design cold pressure is 1 Bar this should rise to no more than 1.5 Bar hot with an extra 150 litre radiator volume.
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I agree ... If at max heating temp the system pressure does not rise much above 2 bar (Worcester actually state 2.6 bar) you don't need an additional expansion vessel irrespective of system capacity.
I have a 440 in a four bedroom house with 15 rads and, with standard installation pressures of 0.75 bar expansion vessel charge and 1 bar system pressure, at max heating temp the pressure never goes above 1.75 bar.
Thus, no need for an additional expansion vessel.
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